Optimum Electroabsorption in Quantum Wells as a Compromise between Exciton Confinement and Linewidth Broadening

The dependence of electroabsorption in GaAs/AlxGa1-xAs multiple quantum well (MQW) structures on well width (Lz) is of considerable importance in optimizing optical modulation devices.1,2 However, a complete characterization including the narrow well width limit has still been lacking. Previously, we have reported that for GaAs/Al0.32Ga0.68As MQWs with 100 Å barriers the maximum obtainable change in the absorption coefficient (Δαmax) increases monotonically with decreasing Lz (from 260 to 50 Å) at the cost of an increasing applied electric field (εmax).2 It was predicted that a peak value would be reached near Lz = 25 Å, being limited at lower Lz by either dielectric breakdown or reduced carrier confinement in the well. In order to determine the complete well width dependence of band edge electroabsorption in GaAs/AlxGa1-xAs MQW structures, our experiments were extended to cover the range from 47 to 17 Å. We demonstrate that this range does indeed contain an optimum value of Lz that provides a peak value in Δαmax. But contrary to our prediction, we shall show that enhanced broadening of the heavy hole excitonic resonance, resulting from phonon scattering at small Lz, is responsible for this result.